The present invention relates to harmonic suppression in dual band mobile phones
Currently mobile Phones are predominantly operated with only a single operating frequency. FIG. 10 shows one realization of the transmitting/receiving operation in a mobile phone being operated with a transmitting frequency, e.g., of approximately 900 MHz for GSM, approximately 1800 MHz for DCS or approximately 1900 MHz for PCS. An antenna 100 is provided to transmit or receive signals and is connected to a transmitter/receiver change over unit or in other words transmitter/receiver switch 102. A transmitter/receiver change over unit 102 comprises a transmitter switch TX and a receiver switch RX. In the receiving mode the receiver switch TX is opened and the receiver switch RX is closed. To the contrary, in the transmitting mode the transmitter switch TX is closed and the receiver switch RX is opened.
As shown in FIG. 10, in the transmitting mode the transmitting signal is outputted from a power amplifier 102 whichxe2x80x94due to its operation near saturationxe2x80x94exhibits a non-linear behaviour such that besides the desired transmitting signal it also outputs harmonics of the transmitting signals. To meet predefined specifications, e.g., the ETSI-GSM-Standard, after the amplification through the power amplifier 104 there is carried out the matching of the output impedance of the power amplifier 104 in a further circuit unit 106 onto a line impedance of typically 50 xcexa9 and in addition a low pass filtering of harmonics.
However, the circuit design shown in FIG. 10 with the increase of the digital mobile telephony is more and more unable to meet the requirements, in particular in rural areas, as an ever increasing number of subscriber faces only a limited number of transmitting frequencies and related transmission channels. Although an increased transmitting frequency, e.g., approximately 1800 MHz for DCS or approximately 1900 MHz for PCS compared to approximately 900 MHz for GSM allows for an increase in the number of transmission channels due to the increased total transmission bandwidth, this is only possible at the expense of a reduced transmission range at the base stations. As a result the number of base stations necessary to completely cover the prespecified area increases. From a practical viewpoint this results in investment costs which are beyond realistic limits.
To the contrary, a combination of technical advantages of the different approaches outlined above seems to be promising, in particular through the provision of cellular dual band networks and dual band mobile phones adapted thereto, i.e. through a combination of the GSM-frequency band with the DCS-and/or PCS-frequency band. FIG. 11 shows a possible circuit design adapted to the related transmission/receiving operation in a dual band mobile phone which is directly based on the approach shown in FIG. 10.
Here, an antenna 200 is connected to two transmitter/receiver change over units 202 and 204. The transmitter/receiver change over unit 202 comprises a transmitter switch TXa and a receiver switch RXa for a carrier frequency lying in the first frequency band. Further, the transmitter/receiver change over unit 204 comprises a transmitter switch TXb and a receiver switch RXb for a carrier frequency lying in a second frequency band. The switches TXa, RXa, TXb and RXb are operated according to the chosen operating frequency, respectively, as explained with reference to FIG. 10. Further, there is provided a diplexer 206 to join the transmission paths to the antenna. Further, there are provided two power amplifiers 210 and 212 as well as related circuit units 214 and 216 adapted to carry the impedance matching and low pass filtering. Alternatively, the two power amplifiers 210 and 212 for the two transmitting frequencies may be equivalently substituted through a single power amplifier with a plurality of output terminals.
The advantage of this direct generalization of the single band transmitter/receiver output circuit shown in FIG. 10 is that the transmission branches for both frequency bands are fully decoupled. Although the power amplifier is operated near saturation such that harmonics of both transmitting signals are generated these may be damped through appropriate dimensioning of the low pass filters TPa and TPb. Nevertheless, these advantages require additional circuit complexity. The additional circuit units not only increase the fabrication costs significantly but also the space requirements for this dual band transmitting/receiving circuit constitute an argument against its realization.
One approach to reduce the fabrication costs and the space requirements is the use of power amplifiers which output transmitting signals in a plurality of frequency bands via a single output terminal or equivalently of power amplifiers having different operation modes. For a combination of, e.g., GSM with a transmitting frequency of approximately 900 MHz and DCS with a transmitting frequency of approximately 1800 MHz the respective output power amounts to approximately 3 W and 1.5 W.
However, as explained with reference to FIGS. 10 and 11 the operation of the power amplifier is near saturation and thus in the first operation mode harmonics are generated at approximately 1800 MHz, approximately 2700 MHz, . . . and further in the second operation mode harmonics are generated at approximately 3600 MHz, etc. Here, regularly harmonics of first and second order are the dominating harmonics.
The result for the operation mode in the first transmitting frequency of, e.g., 900 MHz is that the first harmonic of the transmitting signal at 1800 MHz is not suppressed in the low pass filter TPb in the second transmission branch. Further, also with an opened transmitting switch TXb in the second transmission branch a full decoupling to the antenna is not achieved and therefore, not only the actual transmitting signal is radiated by the antenna but also the first harmonic thereof via the deactivated transmission branch for the second higher transmitting frequency. This occurs to an extent exceeding the limits set by predefined standards. Generally, this problem occurs for power amplifier outputting transmitting signals in a plurality of frequency bands in case the harmonic of the First, lower transmitting frequency is lower than the second, higher transmitting frequency or is identical thereto.
Therefore, the object of the present invention is to effectively suppress harmonics in a dual band mobile phone with a power amplifier outputting transmitting signals in different frequency bands via one output terminal.
According to the invention this object is achieved through a power amplifier output circuit for a dual band mobile radio unit, comprising a first transmitter/receiver change over means to transmit/receive a first transmitting/receiving signal in a first frequency band via an antenna of the mobile radio unit, a second transmitter/receiver change over means to transmit/receive a second transmitting/receiving signal in a second frequency band above the first frequency band via the antenna of the mobile radio unit, wherein a change over means selectively connects a power amplifier to amplify the transmitting signals with the first and second transmitter/receiver change over means, respectively, and an impedance transformation means is provided to transform a turn off impedance of the second transmitter/receiver change over means during the transmission of the first transmitting signal into a band stop characteristic tuned to harmonics of the center frequency f1 of the first frequency band.
According to the present invention the switchable band stop comprises three parts: the change over unit to selectively connect the power amplifier to the transmission branch for the second frequency band, the second transmitter/receiver change over unit and finally an impedance transformation means connected therebetween, respectively. All three components constitute a switchable band stop filter. The two change over units in addition provide essential necessary further functionalities. Only the impedance transformation means is provided exclusively for the implementation of the band stop behaviour.
During the amplification of the transmitting signal in the first frequency band the change over means in the second transmission branch constitutes an open circuit. In addition, the impedance transformation means transforms a turn off impedance of the second transmitter/receiver change over means into a short circuit. This leads to an impedance transmission step in the second transmission branch which enablesxe2x80x94from a practical viewpointxe2x80x94a nearly full reflection of the harmonics to be suppressed.
Overall the concept of a multifunctionality for the circuit units available anyhow in the power amplifier output circuit in interaction with an impedance transformation leads to an overlapping or in other words interleaved provision of different functional sections in the power amplifier output circuit. This enables the desired filtering of harmonics with essentially no additional expenditure.
According to a preferred embodiment of the invention the impedance transformation means is a quarter line or equivalently xcex/4 line being tuned to the double center frequency of the first frequency band.
Since a xcex/4 line may be easily fabricated in the form of a microstrip line or a strip line on a circuit substrate existing and approved circuit designs must only be slightly modified.
According to a further preferred embodiment of the present invention the change over means comprises a diode of the PIN type in each transmission branch, respectively. Further, there is provided an additional diode of the PIN-type as transmitter switch in the second transmitter/receiver change over means.
Thus, the diodes of the PIN type are provided in a series configuration in the second transmission branch during transmission in the second frequency band. Due to the series configuration a biasing of these diodes is necessary only during transmission via the second transmission branch. This may be achieved very easily since the same biasing current is flowing through all diodes of the PIN type and therefore, this biasing current may be controlled using only a single switching transistor. Further, only a single power supply is necessary to provide this biasing current which according to the present invention may also be used as drain power supply of the power amplifier.
Another reason speaking in favour of the series configurations of the diodes of the PIN type is that an inductivity is necessary to decouple the high frequent components of the transmission signals and the power supply for the biasing current. This inductivity is an open circuit for the high frequency components of the transmission signals and a short circuit for the biasing current. Since the inductivity or circuit component may only be fabricated at high costs the integrated supply of the diodes of the PIN type via only a single inductivity for decoupling purposes leads to decreased fabrication costs.
According to yet another preferred embodiment of the present invention there is provided a switchable notch filter in Shunt configuration downstream the transmitter switch of the second transmitter/receiver change over unit. The switchable notch filter comprise a capacitor connected to ground via a diode of the PIN type.
This enables a further improved damping of harmonics. The capacitor of the switchable notch filter together with the parasitic inductivity of the related diode of the PIN type in the activated state constitutes a series resonance circuit to filter harmonics during transmission of the first transmitting signal. Therefore, to achieve a further improved filtering behaviour only in this operation mode the notch filter must be switched or, equivalently, the diode of PIN type must be applied with current.
According to yet another preferred embodiment of the present invention there is provided an impedance matching circuit at the output of the power amplifier. Further, there are provided additional impedance matching circuits downstream the diodes of the PIN type in the change over means to achieve a related further impedance matching in the different transmission branches
This stepwise approach to the impedance matching allows for a simultaneous matching of the output impedance of the power amplifier onto the load impedances specified for the different frequency bands and transmitting powers.